A measurement or test of the type contemplated by the present invention is referred to as a HUM test and is commonly used in testing cable television systems. Indeed, FCC Rules and Regulations 76.605 (7) state that "The peak-to-peak variation in visual signal level caused by undesired low frequency disturbances (hum or repetitive transients) generated within the system, or by inadequate low frequency response, shall not exceed 5 percent of the visual signal level."
The methods and instruments heretofore known for making the HUM test have been unusable for some applications and are restricted for use with special signals for cable systems because they measure amplitude changes of the continuous wave (CW) reference signal which is usually transmitted on the cable along with the standard television signals. There are, however, some cable distribution systems which do not transmit the CW reference signal, wherefore the prior art methods and instruments cannot be used to perform the HUM test on these systems.